When weaving a fabric on a weaving machine, the warp threads are positioned with respect to the level at which a pick thread is introduced in each cycle during the successive weaving cycles. The positions of the warp threads in the successive weaving cycles are in this case determined in such a manner that the weaving process results in a fabric having a predetermined weaving pattern. This positioning of warp threads with respect to the pick introduction level on a weaving machine, referred to as the shed formation, is automatically realized by means of a shed-forming device.
With a known shed-forming device of the jacquard type, each warp thread to be positioned passes through a heddle eyelet of a heddle. At the bottom, each heddle is connected to a retracting spring which exerts a downwardly directed force on the heddle and, at the top, is connected, via a harness cord, to the end of a tackle cord of a tackle system, which end is situated at a higher level. The tackle system comprises two hooks which are displaceable in the vertical direction. The position of these cooperating hooks determines the height of the end of the tackle cord, and thus also of the heddle eyelet and the warp threads passing through the latter.
Each hook can be displaced in the vertical direction by a respective knife. These two knives are driven so as to move in a up and down moving manner in opposite phases with respect to one another. Each hook comprises an elastically deformable portion, referred to as a selection element, which can be brought in a non-selection position or a selection position by actuating an electromagnetic selector. In the selection position, the selection element engages in a fixed hook-shaped projection as a result of which the hook is kept at a fixed height and is thus not caught by its knife. If the selection element is in the non-selection position, it cannot engage in the hook-shaped projection and the hook is caught by a downwardly moving knife. By positioning the hooks in the successive weaving cycles in this manner, a warp thread can be brought into the required successive positions via the tackle cord and the heddle in order to produce a fabric with the desired weaving pattern.
There are also shed-forming devices which comprise a flexible strip which is arranged at a fixed height and can be deformed by an electromagnetic selector and can thus be positioned with respect to a hook. This type of selection elements can optionally be brought into a selection position in which an associated hook can be attached to the selection element and is retained at a fixed height, or can be brought into a non-selection position in which said hook is not retained by the selection element and caught by a knife.
The electromagnetic selectors comprise a solenoid consisting of a core of magnetisable material around which electrically conducting coils are wound and one or more poles. The unit usually has a plastic housing. When an electric current flows through the coils, a magnetic flux is generated so that a deforming magnetic force is exerted on an associated selection element via one or more force-exerting poles of the selector.
By actuating the selector in order to optionally magnetically influence and deform a selection element, the selection element may be brought into the desired selection position or non-selection position.
It is known to provide shed-forming devices with presenting means which push the undeformed selection elements in each weaving cycle slightly in the direction of their respective selector, so that the magnetically influenceable parts of said selection elements are brought closer to the force-exerting poles. This results in an airgap between the magnetically influenceable parts of the selection elements and the selector poles which is less wide, and consequently less electrical energy is required to deform the selection elements.
This results in a reduction in the energy consumption of the numerous selectors in the shed-forming device. This is the case, inter alia, with the shed-forming device according to EP 0 529 025, where the elastically deformable selection elements are pushed in the direction of the selector by a reciprocating knife.
In such a jacquard machine, the chain of cooperating components in a shed-forming device: heddle, retracting spring, harness cord, tackle, selector, hook and hook-guiding means, is present numerous times. All these components have geometric deviations which are the result of, for example, inaccuracies during production and/or not having been accurately positioned and/or having been subjected to mutually different actions of forces, and consequently, the positions of these components are not determined exactly and show relatively large differences between one another. This applies in particular for the position of the various selection elements when these have been deformed in the direction of the selector by the presenting means.
This results in the selection elements being presented in mutually very different positions with respect to the associated selector poles, as a consequence of which the widths of the airgaps between different selection elements and the associated selector poles differ greatly from one another. This inaccurate positioning leads to errors in the selection of selection elements, resulting in weaving errors and weaving machine down time. The reliability of these shed-forming devices thus leaves something to be desired. In order to increase this reliability, it is possible to generate a higher magnetic flux, but this in turn results in an increased energy consumption.